A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as integrated circuits (IC) or image display devices (display devices).
As examples of electronic devices using thin film transistors, mobile devices such as a mobile phone or a laptop computer can be given. For such a mobile electronic device, power consumption that affects continuous operation time is a big problem. Also for a television set which is increasing in size, it is important to suppress the increase in power consumption associated with the increase in size.
In a display device, when image data which is input to a pixel is rewritten, operation of writing the same image data is performed once again even in the case where image data in a period is the same as that in the preceding period. As a result, by performing the operation of writing the same image data a plurality of times, power consumption is increased. In order to suppress such increase in power consumption in a display device, for example, a technique has been disclosed in which an idle period which is longer than a scanning period is set as a non-scanning period every time after image data is written by scanning a screen in the case of displaying a still image (for example, see Patent Document 1 and Non-Patent Document 1).